1. Field of the Invention
The present invention relates to a magneto-optical recording medium that performs recording and reproduction of information with a laser beam utilizing a magneto-optical effect. In particular, the present invention relates to a magnetic domain wall displacement type magneto-optical recording medium that moves a magnetic domain wall of a recorded magnetic domain to expand the recorded magnetic domain for reproduction.
2. Related Background Art
In recent years, as a rewritable high-density recording system, a magneto-optical recording medium and a recording and reproducing device, which write a magnetic domain in a magnetic thin film using thermal energy of a semiconductor laser to record information and read out the recorded information using the magneto-optical effect, are attracting attention. The magneto-optical recording medium is gradually spreading used as a large capacity removable information recording medium in a computer or the like. Recently, data handled by a computer or other information processing apparatus has diversified into various kinds of information such as voices, images, and motion pictures, and a data size necessary for such information is continuously increasing. Therefore, there has been increasing requirement for making recording density of the magneto-optical recording medium higher to further enlarge the capacity of the recording medium.
The magneto-optical recording medium is more excellent in terms of a recording density than other optical recording medium in that it can record a mark finer than a resolution limit of an optical system and can reproduce the recorded mark. For example, Japanese Patent Application Laid-Open No. H7-334877 discloses a magnetic super resolution and reproduction system which is an example of such a magneto-optical recording and reproduction system. Moreover, as a system for recording a magnetic domain finer than a resolution limit of an optical system, there is generally known a magnetic field modulation system with a modulation speed of an external magnetic field sufficiently increased.
In addition, Japanese Patent Application Laid-Open NO. H6-290496 discloses a reproducing method in which a magnetic domain wall displacement layer having a small magnetic domain wall coercive force is provided on an incident side of reproduction light in a magneto-optical recording medium, and a magnetic domain wall of the magnetic domain wall displacement layer is moved to a high temperature side utilizing a temperature gradient in a reproducing spot to thereby expand a magnetic domain in the spot for reproduction. According to this method, even if a recorded mark size is reduced, a signal is reproduced while the magnetic domain is expanded. Thus, the reproduction light can be used effectively, and a resolution can be increased without reducing a signal amplitude.
Moreover, Japanese Patent Application Laid-Open NO. H10-92031 indicates an attempt to improve characteristics of the magneto-optical recording medium disclosed in Japanese Patent Application Laid-Open NO. H6-290496 by devising the magnetic domain wall displacement layer thereof. In other words, a saturation magnetization in a temperature range in which the magnetic domain wall moves is set to 20 emu/cc or less by adjusting a composition of a single magnetic domain wall displacement layer. However, with this constitution in which a single layer is adjusted, a temperature difference ΔT between a lowest temperature Ts, at which an interface magnetic domain wall energy σw is reduced to zero, and a Curie temperature Tc of the magnetic domain wall displacement layer cannot be increased. In order to perform high quality signal reproduction, it is desirable to increase ΔT to 120° C. or more such that a Kerr rotation angle becomes larger. In that case, it is difficult to suppress the saturation magnetization in the entire temperature range in which the magnetic domain wall moves. In addition, an attempt to form a magnetic domain wall displacement layer, which has a composition in the vicinity of a compensation composition, as a multilayer is also performed as another embodiment in Japanese Patent Application Laid-Open NO. H10-92031. However, in that example, a Curie temperature of the magnetic domain wall displacement layer is simply changed stepwise, and the saturation magnetization in the temperature range in which the magnetic domain wall moves is not taken into account.
The reproduction method utilizing magnetic domain wall movement disclosed in Japanese Patent Application Laid-Open NO. H6-290496 is a method particularly excellent in a track recording density among super-resolution systems. In other words, the method has a characteristic that, if a displacement amount of a magnetic domain wall is always constant, a constant reproduced signal amplitude can be obtained regardless of a size of a recorded magnetic domain. However, in addition to a temperature gradient, a magnetostatic force is applied as a force for displacement the magnetic domain wall in a reproducing spot. The magnetostatic force is not always negligibly small. This magnetostatic force is applied from a leaked magnetic field from a memory layer or an optical head and acts in a direction of preventing the movement of the magnetic domain wall in some cases. Thus, in such a case, the displacement amount of the magnetic domain wall is not constant, and stable information reproduction may not be performed.